As this kind of drill head for deep-hole drilling, a divided-blade type as shown in FIG. 5(a) to FIG. 5(c) has been conventionally widely used. This drill head D0 includes a head main body 1 having a substantially cylindrical shape including a hollow portion 10 opened at the proximal end side, one large and one small cutting chip discharge ports 11 and 12 communicating with the hollow portion 10 are formed on a substantially obtuse conical head distal end surface 1a, and a central portion cutting blade tip 2A and a circumferential portion cutting blade tip 2B brazed to the opening side edge along a head radial direction φ of the large cutting chip discharge port 11 and an intermediate portion cutting blade tip 2C brazed to the opening side edge along the same head radial direction φ of the small cutting chip discharge port 12 so as to assume a posture in which cutting blades 20 of the respective cutting blade tips are along the radial direction φ. Guide pads 3A and 3B having arc surfaces to slide-contact with an inner circumference of a cutting hole H indicated by the imaginary line in FIG. 5(b) are brazed respectively to a position radially opposite to the circumferential portion cutting blade tip 2B and an intermediate position in the middle of a route leading to the circumferential portion cutting blade tip 2B in a cutting rotation direction from the aforementioned position on the outer circumferential surface 1b at the distal end side of the head main body 1. The outer end surface of the circumferential portion cutting blade tip 2B consists of a margin portion 21 facing the front edge in the cutting rotation direction and a flank face 22 to be spaced apart from the inner circumference of a cutting hole along the rear side in the cutting rotation direction from the margin portion 21. The cutting blade tips 2A to 2C and the guide pads 3A and 3B are made of a super hard material such as cemented carbide and cermet, etc.
In this drill head D0, a proximal portion 14 side provided with a male thread 13 on the outer circumference is threaded into and mounted on a distal end portion of a hollow boring bar (whose illustration is omitted), and the boring bar is coupled to a driving shaft such as a spindle of a machine tool and rotatingly-driven or otherwise rotates a work material side, thereby cutting the work material by the cutting blades 20 of the cutting blade tips 2A to 2C to form a deep hole. During the deep-hole drilling work, a coolant is supplied into a cutting region through a gap between a cutting hole H and the boring bar at high pressure, made to flow from the cutting chip discharge ports 11 and 12 into the hollow portion 10 together with cutting chips generated in the cutting region, and discharged outside through the inside of the hollow boring bar.
The guide pads 3A and 3B act to increase the cutting accuracy by maintaining a rotational state on a constant axis without runout of the drill head and crushing and smoothing unevenness of an inner circumferential surface of a cutting hole according to cutting by so-called burnishing action in which the guide pads 3A and 3B slide-contact with the inner circumference of a cutting hole H during drilling work to make the inner circumferential surface of the cutting hole receive the cutting reaction force caused by the cutting blade 20 of each cutting blade tips 2A, 2B, and 2C so that the positional relationship between the slide-contact portion and each cutting blade 20 is kept constant. The margin portion 21 is formed of an inclined surface or an arc surface with a width of approximately 0.2 to 1.0 mm and an inclination angle of approximately 1° to 6° with respect to the tip thickness direction, and accordingly, the margin portion is increased in angle at the outer end front edge portion that line-contacts with the inner circumference of a cutting hole H of the circumferential portion cutting blade tip 2B, and becomes hard to chip, and the margin portion functions to stabilize a cutting state by slide-contacting with the inner circumference of a cutting hole H similar to the guide pads 3A and 3B. It is common that guide pads are provided at the same two positions as described above not only in the illustrated divided-blade type but also in a single-blade type drill head including one cutting blade brazed to the opening side edge of a single cutting chip discharge port provided on the head distal end surface and a drill head including a throwaway tip that has a detachable cutting blade whose blade point can be replaced a plurality of times by rotational displacement.
However, in recent years, it has been demanded to further improve the roundness, cylindricality, and straightness, etc., of a cutting hole by further improving the processing accuracy of deep-hole drilling. Therefore, the inventor of the present invention previously proposed this kind of drill head for deep-hole drilling improved in processing accuracy by well-balanced pressure during drilling work by substantially equalizing the axial lengths and aligning the front and rear end positions in the circumferential direction of the margin portion of the cutting blade tip taking charge of cutting a circumferential side and the two guide pads (Patent Document 1).